1. Field of the Invention
The present invention relates to a strain sensor and a method of measuring a strain amount.
2. Description of Related Art
Strain sensors have been used to measure various physical amounts that act on measuring objects, such as displacements, loads, and accelerations. Specifically, the strain sensors measure strains generated on measuring objects and convert the measured strains into physical amounts to determine the physical amounts.
Recently, there have been wishes, regarding the strain sensors, to measure strains generated on the whole structures for the purpose of monitoring of the structures. That is, attention has been riveted to strain sensors that can measure strain fields and can measure the strains generated on measuring objects at each area, rather than conventional strain sensors that measure strain points.
Disclosed is a strain sensor capable of measuring strain fields by measuring an amount of change in emission wavelength at each area using mechanoluminescent elements that vary in emission wavelength depending on strain amounts in response to an excitation light (see, for example, Japanese Unexamined Patent Application Publication No. 2014-115220).
Disclosed is measurement of strain fields using a moire method by attaching a marker having a grating pattern to a measuring object, photographing a displacement of the grating pattern in response to a load with a camera or the like, and comparing image data obtained before and after the application of the load (see, for example, Japanese Unexamined Patent Application Publication No. 2009-264852).
Unfortunately, measurements of strains with nanometer-scale displacements are difficult with the techniques of Japanese Unexamined Patent Application Publication Nos. 2014-115220 and 2009-264852.
Specifically, in order to measure strains with nanometer-scale displacements with the technique disclosed in Japanese Unexamined Patent Application Publication No. 2014-115220, it is necessary to measure picometer-scale variations in emission wavelength. However, it is very difficult to detect such small variations in emission wavelength with existing spectroscopes.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 2009-264852 uses a camera to measure strains, and thus the accuracy of measurement of strains depends on the resolution of the camera. Considering the resolutions of existing cameras, in order to measure strains by overcoming noises, measurement of strains with micrometer-scale displacements is the limit, and it is difficult to measure strains with even smaller nanometer-scale displacements.